NASA has outlined what it could do, and in what time frame, in case a quarter-mile-wide asteroid named Apophis is on a course to slam into Earth in the year 2036. The timetable was released by the B612 Foundation, a group that is pressing NASA and other government agencies to do more to head off threats from near-Earth objects.

The plan runs like this: Eight years from now, if there's still a chance of a collision in 2036, NASA would start drawing up plans to put a probe on the space rock or in orbit around it in 2019. Measurements sent back from the probe would characterize Apophis' course to an accuracy of mere yards (meters) by the year 2020.

If those readings still could not rule out a strike in 2036, NASA would try to deflect the asteroid into a non-threatening course in the 2024-2028 time frame by firing an impactor at it — using this year's
Deep Impact comet-blasting probe
as a model. Experts would start planning for the "Son of Deep Impact" mission even before they knew whether or not it was needed.

The plan is described in a letter attributed to Mary Cleave, NASA's associate administrator for the science mission directorate, as well as a scientific paper by Steve Chesley, an asteroid specialist at NASA's Jet Propulsion Laboratory. The letter was addressed to the B612 Foundation, and B612 made the letter and the paper public on Friday evening.

‘Thorough and thoughtful’ analysis
The B612 Foundation said it was grateful for NASA's "thorough and thoughtful" analysis, which came in response to the foundation's call for a near-term mission to the asteroid, back in June. Former astronaut Rusty Schweickart, the chairman of the California-based foundation's board, said the plan would pose challenges for NASA officials.

"It's certainly a tight schedule," he told MSNBC.com Sunday, "but on the other hand, they're the experts."

Schweickart also noted that Apophis was an unusual case among potentially threatening asteroids, in that it would take a relatively small deflection to eliminate the possibility of a catastrophic collision. "In the typical case, that isn't going to do the job," he said.

Flurry of concern
Apophis, also known as 2004 MN4, stirred up a
flurry of concern
last December when the risk of collision was raised temporarily to as high as 1 out of 40 for the year 2029. With an estimated diameter of 1,300 feet (400 meters), the asteroid could destroy a city if it hit the wrong place on land, or raise a deadly tsunami if it plunged into the ocean.

Fortunately, more precise plotting ruled out a collision in 2029. However, Apophis will still make an extremely close pass — missing Earth by mere tens of thousands of miles. At that distance, Earth's gravitational pull could perturb Apophis' orbit enough to put it on a track to hit during another pass in 2036. Experts say that could happen if, during the 2029 close encounter, the asteroid passes through an outer-space "keyhole" that measures about 2,000 feet (600 meters) across.

In statistical terms, the risk of an impact is now set at 1 in 5,560, based on the uncertainties surrounding Apophis' orbit.

Will it make impact?
Asteroid-watchers may be able to rule out a collision entirely as early as next year, when Apophis is in a good position for further observations. However, the key observations will come in 2013, when astronomers can analyze subtle changes in the asteroid's orbit. If that analysis shows there's still a significant chance of impact in 2036, NASA would send a radio-equipped probe toward a 2019 rendezvous with the asteroid, and collect a year's worth of data about its position.

"With the use of these transponder data, the 2036 impact could be definitively ruled out (or in) by 2020," Cleave said in her letter.

If the impact is "ruled in," NASA would proceed with the deflection probe. "Although the precise method and timeline of a deflection effort cannot be established in this early stage, the recent experience of Deep Impact, which went from initial planning to successful impact on Comet 9P/Tempel 1 in less than seven years, is relevant," Cleave said.

One way or another, NASA would try to push the comet out of a path leading to the 2029 keyhole. The letter explained that "it would be far easier to accomplish a deflection mission prior to the 2029 close approach to avoid any potential 2036 collision."

Of course, chances are that the Apophis affair will turn out like previous asteroid alarms have — with more detailed observations eventually ruling out the threat. Even in that case, NASA may decide to go ahead with the radio probe or the deflection probe anyway, for the sake of science rather than planetary survival.

Concerns remain
Schweickart said the B612 Foundation — which takes its name from an asteroid mentioned in Antoine de Saint-Exupery's "The Little Prince" — still had concerns about threats from near-Earth objects in general, and about Apophis in particular.

He worried that the funding might not be available to make high-quality radar readings of Apophis by 2013 — particularly readings from the Arecibo radio telescope in Puerto Rico, the world's largest single radio dish. "It's no secret that Arecibo is fairly precarious right now, and especially the radar function, because that is not needed for the bulk of radio astronomy," Schweickart said.

The B612 Foundation said the National Science Foundation should make sure there is reliable radar capability "to support early warning of pending NEO [near-Earth object] impacts and rational deflection mission planning."

Schweickart said NASA should also boost research into advanced propulsion methods that might come into play for deflecting near-Earth objects — such as Project Prometheus, the nuclear propulsion program that was recently pared back.

Who should be in charge?Finally, Schweickart and the B612 Foundation said the responsibility for protecting Earth from hazardous asteroids and comets should be officially assigned to a capable U.S. government agency. That agency might turn out to be NASA or the Department of Defense, Schweickart said, but "from a bureaucratic point of view, the candidate might be Homeland Security."

Schweickart said the cost of fending off dangerous near-Earth objects would be far less than the cost of cleaning up after this year's hurricanes — and would produce spin-offs in the form of scientific insights as well as next-generation propulsion and power systems.

"The development of that technology is not like the development of a levee around New Orleans, which serves only one function," he said.